Silicon is the most common substrate material in semiconductor devices, particularly semiconductor integrated circuits (IC), microelectronics devices, and in accurate measurement devices. Silicon substrates are used widely because silicon wafers of high purity can be industrially manufactured inexpensively. In addition, a highly chemically stable oxide film (SiO2) is formed on the silicon substrate, and this results in a simple insulated separation of elements in high integration devices.
For example, in manufacturing ICs by the batch system, several devices are formed on top of a single silicon wafer in an electrically insulated and isolated state. The insulated separation required for this device is achieved by forming an oxide film of approximately 0.3–1.0 micron on top of the wafer.
An example of a precision device includes a ring-shaped vibrating gyroscope (for example, Japanese Patent Publication Number 10-267667). The ring-shaped vibrating gyroscope is a type of vibrating gyroscope in which a suspended ring-shaped vibratory resonator is provided with an elliptical vibration. Unlike a piezoelectric gyroscope which is also a vibrating gyro, the ring-shaped vibrating gyroscope does not have a fixed support point sensitive to external stress. Because the elliptical vibration is affected less by the external stress, highly accurate angular velocity measurements can be achieved.
FIG. 1 shows a silicon substrate for a ring-shaped vibrating gyroscope. FIG. 2 shows a wiring pattern provided on top of the ring.
Referring to FIG. 1, a silicon substrate 1 has a construction in which a ring 1-1 positioned in the center is suspended by eight suspensions 1-2. Referring to FIG. 2, silicon substrate 1 (ring 1-1 in the example in FIG. 2) is a monocrystal silicon wafer 2, which is thinner than those normally used in integrated circuits such as CPUs and the like, and has an insulating film 3. The ring-shaped resonator is created by the following steps: after an Al—Si conductive film for wiring is formed on top of insulating film 3 by sputtering, a track 4 (wiring pattern) is formed by photolithography and chemical etching, and in addition, ring 1-1 and suspensions 1-2 are formed by ICP (inductively coupled plasma) etching.
There is a circumferential flow of alternating current in track 4 on ring 1-1 and suspensions 1-2. A magnetic field is applied to track 4 in the vertical direction by a magnetic circuit (not shown). As a result, track 4 deforms into an elliptical shape by Lorentz force (this is vibration mode 1). By adjusting the phase of the current flowing through suspension 1-2, angular velocity is given to the elliptical vibration of ring 1-1. When this occurs, Coriolis force acts against vibration mode 1, and a vibration mode 2 positioned at a displacement of 45 degrees from vibration mode 1 is generated. By monitoring the node of vibration mode 1 generated by vibration mode 2, the angular velocity is obtained.
Bias drift is used as an index for measuring the performance of a gyroscope. Bias drift is the change in the signal from the gyroscope indicating that the ring is rotating even though in reality the ring is stationary. A smaller bias drift indicates better performance of the gyroscope. The unit for bias drift is the apparent angular velocity detected per unit time (usually around 1 hour) [(deg/sec)/hr]. Reasonable values for bias drift vary depending on the purpose, but for example, in gyroscopes used for oscillation control, this is [0.05 (deg/sec)/hr] or lower.
Bias drift is a very important index for ensuring an acceptable performance for the purposes of the gyroscope. Even gyroscopes made using the same materials can have a range in values for bias drift. By studying the reason for this, the present inventors discovered that samples with large bias drifts had comparatively larger grain size in the conductive film used for wiring.
Referring to FIG. 3, the metal structure of a conductive film used for wiring of the vibrating gyroscope is shown. FIG. 3(a) is a structural diagram of a conductive film for wiring in a sample with a small bias drift of 0.03 (deg/sec)/hr. FIG. 3(b) is a structural diagram of a conductive film for wiring of a sample with a large bias drift of 0.12 (deg/sec)/hr. These figures are traces of EBSP images of sections of the conductive films for wiring. Referring to FIG. 3, the crystal grains of the sample with a small bias drift was smaller compared to samples with a large bias drift.
In general, conductivity of the wiring used in semiconductor devices is better when the grain size is large. The main reason for this is that when the grain size of the wiring is small, the grain boundary is increased, and this acts to resist and interfere with the movement of free electrons.